Electron beam tube and method of adjusting the electrode spacing of an electron gun therein

ABSTRACT

A novel electron beam tube and a method of adjusting the electrode spacing of an electron gun therein utilize an expansion member to support the cathode cylinder of the gun within the electron gun assembly of the tube. With this method, the proper control grid-cathode spacing for cutoff of the electron gun at desired operating voltages is fixed after the tube is constructed and while the gun is operating under normal conditions.

United States Patent Brown 5] Feb. 22, 1972 [54] ELECTRON BEAM TUBE AND METHOD OF ADJUSTING THE ELECTRODE SPACING OF AN ELECTRON GUN THEREIN [72] Inventor: Martin Kamp Brown, Lancaster, Pa.

[73] Assignee: RCA Corporation [22] Filed: June 16, 1969 [21] Appl. No.: 833,615

521 U.S.Cl ..29/25.16,29/25.15,316/19 [51] lnt.Cl. ..H01j9/18,l-101j9/36 [5s FieldoiSearch ..3l6/l9,17;29/25.l,25.1l,

[56] References Cited UNITED STATES PATENTS 3,210,145 10/1965 ll/Ill Fyler 2,174,853 10/1939 Bowie ..29/25.16 2,338,336 1/1944 Koch ..29/25.16 X 2,413,267 12/1946 Trumbull et al. .....29/25.16 X 3,387,166 .....29/25.l6 X

6/1968 Kraner et al.

Primary Examiner-.1 ohn F. Campbell Assistant Examiner-Richard Bernard Lazarus AttorneyGlenn H. Bruestle [57] ABSTRACT A novel electron beam tube and a method of adjusting the electrode spacing of an electron gun therein utilize an expansion member to support the cathode cylinder of the gun within the electron gun assembly of the tube. With this method, the proper control grid-cathode spacing for cutoff of the electron gun at desired operating voltages is fixed after the tube is constructed and while the gun is operating under normal conditions.

9 Claims, 4 Drawing Figures mmtufia zzlm 3.643.299

Marl/n Kamp Brown AGENT ELECTRON BEAM TUBE AND METHOD OF ADJUSTING THE ELECTRODE SPACING OF AN ELECTRON GUN THEREIN BACKGROUND OF THE INVENTION This invention relates to the manufacture of cathode-ray or electron beam tubes and, particularly, to improvements in the method of adjusting the electrode spacings of cathode-ray tube electron guns.

A conventional cathode-ray tube comprises an electron gun assembly containing at least one electron gun for generating an electron beam. A typical electron gun comprises a cathode cylinder, the closed end of which is coated with an electronemissive coating, and a plurality of axially aligned electrodes including a control or first grid and a screen or second grid spaced from the cathode cylinder in the order named. The grids and cathode of the gun may be supported within the electron gun assembly by means of insulating rods.

In the manufacture of cathode-ray tubes whose operating characteristics are to fall within certain desirable limits, it is necessary to accurately control the cutoff voltages of the electron guns incorporated in such tubes. The cutoff voltage of a cathode-ray tube electron gun is normally that voltage which must be applied to the control grid, while specified voltages are impressed on the remaining electrodes of the gun, to either just extinguish the undeflected focused light spot on the faceplate screen or just reduce the cathode current of the gun to some specified low value, usually about 1 microarnpere or less. Alternatively, the voltage on the screen grid may be varied, while specified voltages are impressed on the remaining electrodes of the gun, to obtain the cutoff condition.

Ordinarily, the cutoff voltage of a cathode-ray tube electron gun is predetermined by setting the spacings between the various electrodes of the gun before incorporating the electron gun assembly in the tube. For example, a calculation is first made of the theoretical control grid-cathode spacing for a specified control grid screen grid spacing in the electron gun. A subassembly of the gun comprises the properly spaced grids held in axial alignment with the cathode sleeve in which the cathode cylinder is later installed. The subassembly is placed in a suitable mechanical jig, and an air nozzle is inserted through the apertures of the grids and directed at the closed end of the cathode cylinder. The back pressure of the air on the electron-emissive coating is measured by a gauge calibrated in terms of the control grid-cathode spacing. Beginning with a slightly oversized spacing, the cathode cylinder is moved closer to the control grid of the gun assembly until the appropriate measure corresponding to the previously calculated spacing is obtained. The cathode cylinder is then welded in place at this position.

An alternative method of positioning the cathode cylinder of a cathode-ray tube electron gun comprises the steps of calculating and then measuring, by means of a capacitance bridge, the direct capacitance of the cathode to the screen grid. However, this method is usually less accurate than that employing an air pressure set.

With each of these methods, care is taken to include in the calculation of the required spacing or capacitance the effects of eventually operating the electron gun within the cathoderay tube. For example, mechanical movement and distortion of the gun elements occur when the cathode is heated. Nevertheless, large variations are encountered in the cutoff characteristics of cathode-ray tubes made by these methods. Typically, the control grid cutoff voltage of a cathode-ray tube electron gun may vary between 50 volts and l 50 volts. Alternatively, at cutoff, the voltage on the screen grid of a cathode-ray tube electron gun may vary between 155 volts and 385 volts for a given control grid voltage.

SUMMARY OF THE INVENTION A novel electron beam tube includes an electron gun comprising means operable from outside the tube envelope for adjusting the grid-cathode spacing of the gun, during operation after the tube has been assembled and evacuated. Preferably, the method of the invention utilizes an expansion member coupled between a fixed support and the open end of the cathode cylinder of a cathode-ray tube electron gun to accurately adjust the electrode spacing of the gun. Before the gun is sealed within the tube envelope, the cathode cylinder is positioned so that the initial spacing between the control grid of the gun and the closed end of the cathode cylinder is slightly oversized. The final grid-cathode spacing is determined in the completed tube, after the electron-emissive coating on the cathode cylinder is activated and while typical operating voltages are applied to the electrodes of the gun. The spacing between the control grid and the cathode may be reduced in the completed tube by heating the expansion member of the gun by means of a focused beam of radiation, such as a laser beam, transmitted through the tube envelope from an appropriate source, such as a pulsed ruby laser, located outside the envelope. The spacing is reduced until either light-output observations or beam-current measurements show that the electron gun is just at cutoff. The cathode cylinder may then be welded in place in the electron gun by means of a laser beam transmitted through the tube envelope from a pulsed ruby laser located outside the envelope and focused on the cathode sleeve. Thus the electrode spacing of the electron gun can be accurately controlled, and the variations in cutoff voltage usually encountered with cathode ray tube guns can be substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in greater detail in connection with the accompanying sheet of drawings, wherein:

FIG. 1 is aside view, partly in axial section, of a typical cathode-ray tube including an electron gun assembly containing at least one electron gun;

FIG. 2 is an enlarged axial-section view of the electron gun of FIG. 1, showing an expansion section and also depicting means for heating and welding parts of the gun; and

FIGS. 3 and 4 are sectional views of alternative forms of the expansion section of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 depicts a typical cathode-ray tube, for example a color television picture tube, comprising a glass envelope 1. The envelope 1 comprises a curved faceplate 3 having a phosphor screen on the inner surface thereof and joined at its periphery to the larger end of a funnel 5. The smaller end of the funnel 5 forms a hollow cylindrical neck 7 within which is positioned an electron gun assembly 9. The assembly 9 may contain three electron guns 1 1 (only one of which is shown in FIG. 1 The guns 11 may be arranged in a conventional triangular fashion such that each of the gun axes is parallel to the longitudinal axis of the tube.

FIG. 2 is an enlarged view of a portion of the electron gun 11 shown in FIG. 1. The gun 11 comprises a plurality of insulatedly coupled electrodes including a screen grid 13 and a control grid 15. Each of the grids 13 and 15 is secured to a plurality of insulative posts or rods 17 (only one of which is shown in FIG. 2). The posts 17 also serve to support a flange 18 of a metal cathode sleeve 19 within which is held, in sliding electrical contact therewith, a cathode cylinder 21. The cylinder 21 has a closed end 22 externally coated with an electron-emissive material 23 and an open end 24 attached, as by welding, to one end of an expansion member 25. The expansion member may be a hollow cylindrical bellows, as shown, made of a high expansion metal such as Nichrome. The other end of the expansion member 25 is welded to a support plate 27 secured, in turn, to the posts 17. The position of the plate 27 is fixed so that the initial spacing between the control grid 15 and the cathode cylinder coating 23 is slightly oversized with respect to the normal spacing. The initial control gridcathode spacing may be several thousandths of an inch wider than normal. A heater coil 29 is supported within the cathode cylinder 21 by means of straps 30 secured to the insulative posts 17.

Following its construction, the electron gun assembly 9 is mounted on a standard cathode-ray tube stem 31, shown in FIG. 1. The stem 31 comprises an extended tubulation (not shown) and a plurality of electrical lead-ins 35 connected appropriately to the electrodes, including grids 13 and 15, the cathode 21, and the ends of the heater 29 of each of the guns 11. The stem 31 is in turn sealed to the open end of the neck 7 of the tube envelope 1, thereby completing the envelope 1, in such manner that each of the guns 11 in the assembly 9 faces forwardly toward the faceplate 3. The tubulation is then connected to a vacuum pumping system (not shown), and the tube is exhausted to a high vacuum sufficient for processing each of the guns 11 in a normal manner.

Consider again the electron gun 11 shown in FIGS. 1 and 2. After the final activation of the cathode coating and, for example, while the tube is still on exhaust, a set oftypical operating voltages is impressed upon the elements ofthe gun I 1including the grids 13 and 15, the cathode 21, and the heater 29by means of the lead-ins 35 extending outside the tube envelope 1, the voltage on the control grid being the desired cutoff voltage of the gun 11. For these operating voltages in combination with the oversized control grid-cathode spacing, the electron gun 11 is below cutoff. That is, either no visible spot is produced on the screen of the faceplate 3, the voltage of which is set at a typically high operating value, or the cathode current of the gun 11 is less than a specified cutoff value. The expansion member of the gun 11 is then heated by energy transmitted through the tube neck 7, from a source 37 located outside the envelope, whereby the member 25 is caused to expand toward the control grid 15. The member 25 is heated for a time sufficient to reduce the control gridcathode spacing to that critical spacing for which the electron gun 11 is just at cutoff. That is, either a visible spot is just produced on the screen of the faceplate 3 or the cathode current ofthe gun 11 is just at the specified cutoff value.

The expansion member 25 may be heated by means of a focused beam of radiation, such as a laser beam, directed at the member 25 and transmitted through the tube neck 7 from an appropriate source 37, such as a pulsed ruby laser, located outside the envelope. As shown in FIG. 2, the laser 37 may comprise a ruby crystal 39 excited by a usual optical exciting tube 41, the tube 41 surrounding the crystal 39 in a helical fashion. The tube 41 is connected to a suitable laser power supply 43 the energy pulses from which cause the tube 41 to become periodically illuminated. In accordance with known laser action, the periodic illumination of the tube 41 causes the crystal 39 to emit a fine coherent light beam 44 from one end of the crystal 39. This beam contains visible energy at a wavelength of around 6,943 angstroms, to which radiation the glass envelope 1 surrounding the cathode ray tube electron gun assembly 9 is highly transparent.

Thus, as described above, the expansion member 25 of the electron gun 11 is heated to produce the critical control gridcathode spacing corresponding to the desired cutoff voltage of the gun 11. This spacing is maintained while the sliding cathode cylinder 21 is welded to the support sleeve 19 of the gun 11 by energy also transmitted through the tube neck 7, from a source located outside the envelope. At least two welds may be necessary to secure the final position of the cathode cylinder 21. The cathode cylinder 21 may be welded to the sleeve 19 by means of a laser beam directed at the weld points and transmitted through the tube neck 7 from a pulsed ruby laser 45 located outside the envelope. As shown in FIG. 2, the laser 45 comprises a ruby crystal 47 excited by an exciting tube 49 connected to an appropriate power supply 51. In the same manner described above, the crystal 47 emits a fine coherent light beam 48 from one end of the crystal 47, the beam containing energy at a wavelength of around 6,943 angstroms. For very fine focusing of the beam onto the weld points, the laser 45 may employ a focusing lens 53. A focusing lens is not employed by the first laser 37 because heating of the expansion member 25 does not require concentration of the energy from the laser beam 44.

The laser 45 used for welding may be essentially the same as the laser 37 used for heating, except that the respective power supplies 57 and 43 will differ. Laser welding is achieved by surface heating and thermal conduction through the metal being welded. The power density at the surface of the metal must be sufficient to raise the temperature of the surface up to the fusion point but below the vaporization point. Also, the energy must be supplied in a time short enough to permit lo calized heating but long enough to permit penetration of the molten zone. The power supply 51, therefore, must accurately control the power and the pulse duration of the welding laser beam. Laser heating, on the other hand, places relatively simpler demands on the power supply 43.

The uses of a laser beam to heat and weld metal members through a glass wall are known in the art. For example, in an article by R. D. Haun, Jr., entitled Laser Applications, appearing in IEEE Spectrum, volume 5, number 5, pages 82 to 92, May 1968, it is stated:

Welding can be done in air, vacuum, or a controlled atmosphere. The laser beam can enter the enclosure through a transparent window. The article also reports that several faulty electron tubes were repaired by welding 0.76-millimeter-diameter Kovar wires to 0.25 centimeterthick steel tabs through the glass vacuum envelope.

Thus, as described above, the cathode cylinder 21 is welded to the support sleeve 19, whereby the proper control gridcathode spacing is fixed, while the electron gun 11 is operating under normal conditions. The electrode spacing of the gun 11 is thereby accurately determined. The electrode spacing of the other guns 11 in the electron gun assembly 9 may be similarly determined.

The adjustment of the electrode spacing of the gun 11 may be performed while the tube is connected to the vacuum pumping system as described above. Although prior to this adjustment the tube has been evacuated, that is, exhausted to a high vacuum sufficient for sealing the tube; action ofthe heap ing and welding laser beams 44 and 48, respectively, releases some unwanted gases within the tube envelope 1. These gases are removed by the vacuum pumping system and the evacuation is completed. Then the exhaust tubulation is tipped or sealed off, and the usual base 55 is assembled and cemented to the stem 31, as shown in FIG. 1. Alternatively, the tube may contain sufficient gettering means (not shown) to maintain the high vacuum within the envelope and the adjustment of the electrode spacing may be performed after the evacuation of the tube is completed and the tube is sealed off. Therefore, in the claims, the term evacuated is intended to cover either complete evacuation and seal off or substantially complete evacuation.

As shown in FIG. 2, the expansion member of the gun 11 may comprise a hollow cylindrical bellows 25 attached to and mechanically connecting the support arm 27 and the open end 24 of the cathode cylinder 21. Alternatively, as shown in FIG. 3, the expansion member may comprise a bowed strip 57, made ofa suitable high expansion metal, the ends of which are attached, as by welding, to the support plate 27 and the midportion of which abuts the open end 24 of the cathode cylinder 21. The strip 57 may then be heated as described above, e.g., by means of a laser beam directed at the expansion member and transmitted through the tube neck from a pulsed ruby laser located outside the envelope, whereby the strip 57 is caused to expand upwards and push the cathode cylinder 21 closer to the control grid (not shown in FIG. 3). The method of determining the cathode-grid spacing of the gun 11 is thereafter the same as described above. A further alternative form of the expansion member is shown in FIG. 4, wherein the expansion member comprises a bimetallic strip 59, made typically of a lower thermal expansion first layer 61 welded to a higher thermal expansion second layer 63. The strip 59 is positioned such that one end of each of the layers 61 and 63 is attached to a fixed support member, such as one of the electrical lead-ins 35, and the other end of the first layer 61 abuts the open end 24 of the cathode cylinder 21. Heating of the expansion member, as described above, then causes the strip 59 to bend upwards and push the cathode cylinder 21 closer to the control grid (not shown in FIG. 4). Therefore, in the claims, the term coupled" is intended to cover either attachment or abutting of the expansion member and the open end of the cathode cylinder.

It should also be understood that the invention is not limited to the uses of ruby or other laser beams to heat the expansion member and weld the cathode to the cathode support. For example, in an alternative embodiment (not shown), the expansion member may be surrounded inside the tube envelope by a heating coil made of a suitable refractory metal, the ends of which coil may be welded to a set of electrical lead-ins extending outside the tube envelope. The lead-ins, in turn, may be connected to an appropriate external power supply the energy from which is thereby transmitted to the heating coil. The expansion member is then heated by the heating coil. In a further alternative embodiment (not shown), the expansion member may be connected directly to a set of electrical lead-ins extending outside the tube envelope and thereby heated directly by the energy of an external power supply.

I claim:

1. In the manufacture of an electron beam tube comprising an evacuated envelope containing an electron gun comprising a cathode, at least one grid adjacent to said cathode, means mounting said grid in a fixed position in said gun, and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; the method of determining the cathode-grid spacing of said gun comprising the steps of:

a. adjusting the spacing between said cathode and said grid to a value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated; and

b. fixing said cathode-grid spacing in said gun at said value,

during said operation.

2. In the manufacture of a cathode-ray tube including an evacuated envelope containing an electron gun comprising: a cylindrical metal cathode; at least one grid adjacent to said cathode, said cathode having an electron-emissive coating on the end thereof adjacent to said grid; means mounting said grid in a fixed position in said gun; and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; said cathode mounting means comprising a hollow cylindrical support in which said cathode is slidable, a second support adjacent to the other end of said cathode, and a metal thermal expansion member coupled between said other end of said cathode and said second support, said supports being mounted in fixed positions such that the initial spacing between said cathode and said grid is slightly oversized, said expansion member and the contact area between said cathode and said first-named support being separately responsive to energy from external source means; the method of determining the cathode-grid spacing of said gun comprising the steps of:

a. directing said energy from said external source means to expand said expansion member and move said cathode closer to said grid, thereby adjusting said cathode-grid spacing to a value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated; and

b. directing said energy from said external source means to secure said cathode to said first-named support, thereby fixing said cathode-grid spacing at said value in said gun, during said operation.

3. In the manufacture of a cathode-ray tube as set forth in claim 2, wherein said envelope is transparent to focused radiation; and said method of determining the cathode-grid spacing of said gun comprises the step of heating said expansion member by means of a focused beam of said radiation directed at said member from a suitable source located outside said envelope, for a time sufficient to expand said member such that said cathode-grid spacing is reduced to said desired value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated.

4. In the manufacture of a cathode-ray tube as set forth in claim 2, wherein said envelope is transparent to focused radiation; and said method comprises the step of welding said cathode to said cathode support by means of a focused beam of said radiation directed at points of contact from an appropriate source located outside said envelope, thereby fixing said cathode-grid spacing in said gun, during said operation.

5. The method of claim 1, wherein said expansion member comprises a hollow cylindrical bellows attached to said second support and to said other end of said cathode.

6. The method of claim 1, wherein said expansion member comprises a metallic strip attached to said second support and abutting said other end of said cathode.

7. The method of claim 3, wherein said expansion member is heated by means of laser beam radiation directed at said member from a laser source located outside said envelope, said envelope being transparent to said laser beam radiation.

8. The method of claim 4, wherein said cathode is welded to said cathode support by means of laser beam radiation directed at said points of contact from a laser source located outside said envelope, said envelope being transparent to said laser beam radiation.

9. In the manufacture of an electron beam tube including an evacuated envelope containing an electron gun comprising a cathode, at least one grid adjacent to said cathode, means mounting said grid in a fixed position in said gun, and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; the method comprising the steps of:

a. inserting and mounting said gun within said envelope;

b. evacuating and sealing off said tube;

c. then applying operating voltages to electrodes of said gun with a predetermined cutoff voltage applied between said cathode and said grid;

d. then moving said cathode relative to said grid, thereby adjusting the spacing therebetween, while said voltages are applied, until a beam cutoff condition is achieved; and

e. then fixing the position of said cathode in said gun relative to said grid in said cutoff condition.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 264 299 Dated 22 February 1972 Inve Martin Kamp Brown It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 5, line 1, change "1" to 2 Claim 6, line 1, change "1" to 2 Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

ROBERT GOT'ISCHALK At testing Officer FORM (10-69) USCOMM-DC 60376-P69 k U,5. GOVERNMENT PRINTING OFFICE 1 969 0-355-33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,643,299 I Dated 22 February 1972 lnv fl Martin Kamp Brown It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 5, line 1, change "1" to 2 Claim 6, line 1, change "1" to 2 Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK At testing Officer Commissioner of Patents FORM PO-105O (10-69) USCOMM-DC 60376-P69 V U.Sv GOVERNMENT PRINTING OFFICE: 1969 0-356-334 

1. In the manufacture of an electron beam tube comprising an evacuated envelope containing an electron gun comprising a cathode, at least one grid adjacent to said cathode, means mounting said grid in a fixed position in said gun, and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; the method of determining the cathode-grid spacing of said gun comprising the steps of: a. adjusting the spacing between said cathode and said grid to a value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated; and b. fixing said cathode-grid spacing in said gun at said value, during said operation.
 2. In the manufacture of a cathode-ray tube including an evacuated envelope containing an electron gun comprising: a cylindrical metal cathode; at least one grid adjacent to said cathode, said cathode having an electron-emissive coating on the end thereof adjacent to said grid; means mounting said grid in a fixed position in said gun; and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; said cathode mounting means comprising a hollow cylindrical support in which said cathode is slidable, a second support adjacent to the other end of said cathode, and a metal thermal expansion member coupled between said other end of said cathode and said second support, said supports being mounted in fixed positions such that the initial spacing between said cathode and said grid is slightly oversized, said expansion member and the contact area between said cathode and said first-named support being separately responsive to energy from external source means; the method of determining the cathode-grid spacing of said gun comprising the steps of: a. directing said energy from said external source means to expand said expansion member and move said cathode closer to said grid, thereby adjusting said cathode-grid spacing to a value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated; and b. directing said energy from said external source means to secure said cathode to said first-named support, thereby fixing said cathode-grid spacing at said value in said gun, during said operation.
 3. In the manufacture of a cathode-ray tube as set forth in claim 2, wherein said envelope is transparent to focused radiation; and said method of determining the cathode-grid spacing of said gun comprises the step of heating said expansion member by means of a focused beam of said radiation directed at said member from a suitable source located outside said envelope, for a time sufficient to expand said member such that said cathode-grid spacing is reduced to said desired value producing cutoff of the electron beam from said gun at a desired grid voltage, during operation after said tube has been assembled and evacuated.
 4. In the manufacture of a cathode-ray tube as set forth in claim 2, wherein said envelope is transparent to focused radiation; and said method comprises the step of welding said cathode to said cathode support by means of a focused beam of said radiation directed at points of contact from an appropriate source located outside said envelope, thereby fixing said cathode-grid spacing in said gun, during said operation.
 5. The method of claim 1, wherein said expansion member comprises a hollow cylindrical bellows attached to said second support and to said other end of said cathode.
 6. The method of claim 1, wherein said expansion member comprises a metallic strip attached to said second support and abutting said other end of said cathode.
 7. The method of claim 3, wherein said expansion member is heated by means of laser beam radiation directed at said member from a laser source located outside said envelope, said envelope being transparent to said laser beam radiation.
 8. The method of claim 4, wherein said cathode is welded to said cathode support by means of laser beam radiation directed at said points of contact from a laser source located outside said envelope, said envelope being transparent to said laser beam radiation.
 9. In the manufacture of an electron beam tube including an evacuated envelope containing an electron gun comprising a cathode, at least one grid adjacent to said cathode, means mounting said grid in a fixed position in said gun, and means mounting said cathode in said gun for movement and then fixed positioning relative to said grid; the method comprising the steps of: a. inserting and mounting said gun within said envelope; b. evacuating and sealing off said tube; c. then applying operating voltages to electrodes of said gun with a predetermIned cutoff voltage applied between said cathode and said grid; d. then moving said cathode relative to said grid, thereby adjusting the spacing therebetween, while said voltages are applied, until a beam cutoff condition is achieved; and e. then fixing the position of said cathode in said gun relative to said grid in said cutoff condition. 